Photovoltaic (PV) devices (also called solar cells) are now widely used to convert incident sunlight into usable electrical energy, which can supply an external load of any desired type. The cells are provided with electrodes that are configured to be connected to wires through which the generated electricity flows to the load. Of course, it is desired that the generation and transmission of energy be done as efficiently as possible, to maximize the amount of the incident solar energy that can be captured and turned into usable electrical energy. Technologists have devoted attention in recent years to many ways of increasing that efficiency. Solar-powered photovoltaic systems are considered to be environmentally beneficial in that they reduce the need for older forms of generation, such as burning fossil fuels in conventional electric power plants.
Conventional solar cells are based on a structure that includes a junction between semiconducting materials with different majority-carrier conductivity types, such as a p-n junction formed between an n-type semiconductor and a p-type semiconductor. More specifically, crystalline Si photovoltaic cells are typically made by adding controlled impurities (called dopants) to purified silicon, which is an intrinsic semiconductor. Dopants from IUPAC group 13 (e.g., B) are termed “acceptor dopants” and produce p-type material, in which the majority charge carriers are positive “holes,” or electron vacancies. Dopants from IUPAC group 15 (e.g., P) are termed “donor dopants” and produce n-type material, in which the majority charge carriers are negative electrons. Dopants may be added to bulk materials by direct inclusion in the melt during silicon crystal growth. Doping of a surface is often accomplished by providing the dopant at the surface in either liquid or gaseous form, and then thermally treating the base semiconductor to cause the dopants to diffuse inward. Ion implantation, possibly with further heat treatment, is also used for surface doping.
The cell structure includes a boundary or junction between p-type and n-type Si. When the cell is illuminated by electromagnetic radiation of an appropriate wavelength, such as sunlight, a potential (voltage) difference develops across the junction as the electron-hole pair charge carriers migrate into the electric field region of the p-n junction and become separated. The spatially separated charge carriers are collected by electrodes in contact with the surfaces of the semiconductor. The cell is thus adapted to supply electric current to the connected electrical load. Since sunlight is almost always the light source, photovoltaic cells are commonly known as “solar cells.”
In the commonly used planar p-base configuration, a negative electrode is located on the side of the cell that is to be exposed to a light source (the “front,” “light-receiving,” or “sun” side; a positive electrode is located on the other side of the cell (the “back” or “non-illuminated” side). Cells having a planar n-base configuration, in which the p- and n-type regions are interchanged from the p-base configuration, are also known.
Both electrodes are normally provided by suitable metallizations, i.e., thin layers of electrically conductive metal situated on some or all of one or both surfaces of the cell. Most commonly, these electrodes are provided on opposite sides of a generally planar cell structure. Conventionally, they are produced by applying suitable conductive metal pastes to the respective surfaces of the semiconductor body and thereafter firing the pastes. For example, U.S. Pat. No. 8,497,420 discloses a method of manufacturing a solar cell electrode comprising steps of: applying onto a semiconductor substrate a conductive paste comprising (i) a conductive powder such as Ag, (ii) a lead-tellurium-oxide based glass frit, (iii) ethyl cellulose as an organic polymer, (iv) suitable thixotropes and surfactants; and (v) a solvent comprising predominantly 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate; and firing the conductive paste to produce an electrode suitable for devices such as solar cells.
Most photovoltaic cells are fabricated with an insulating layer on their front side to afford an anti-reflective property that maximizes the utilization of incident light. However, in this configuration, the insulating layer normally must be removed to allow an overlaid front-side electrode metallization to make contact with the underlying semiconductor surface. Conductive metal pastes appointed for fabricating front side electrodes typically include a glass frit and a conductive species (e.g., silver particles) carried in an organic medium that functions as a vehicle for printing. The electrode may be formed by depositing the paste composition in a suitable pattern (for instance, by screen printing) and thereafter firing the paste composition and substrate.
The specific formulation of the paste composition has a strong but highly unpredictable effect on both the electrical and mechanical properties of electrodes constructed therewith. To obtain good electrical characteristics for the finished cell (e.g., high light conversion efficiency and low resistance), the composition must penetrate or etch fully through the anti-reflective layer during firing so that a good electrical contact is established, but without damaging the underlying semiconductor.
Ideally, the electrode has high electrical conductivity and a low-resistance connection to the underlying device to minimize the loss of efficiency from ohmic heating within the cell. In addition, it is desirable for the total area of the electrode to be as small as possible to avoid the loss of efficiency that results from shadowing of the incident light on the light-receiving surface. Ordinarily, these requirements necessitate a structure that includes plural fine conductive lines. The conductivity of the lines is improved by increasing their cross-sectional area in the plane perpendicular to their length. But to minimize shadowing, the fired line should be high but narrow. However, with existing paste compositions, it has proven difficult both to form such lines by screen printing and to prevent excessive line spreading during firing. It is further desired that a strongly adhering bond between the electrode and the substrate upon firing is formed. Still further, it is desirable that the vehicle is completely removed during firing, so that there is no residue that degrades the conductivity of the electrode. With many conventional paste compositions, it thus has not proven possible to reliably fire the printed wafers so that good adhesion and electrical properties are obtained in combination.
Although various methods and compositions useful in forming devices such as photovoltaic cells are known, there nevertheless remains a need for compositions that permit fabrication of patterned conductive structures that provide improved overall device electrical performance and that facilitate the rapid and efficient manufacture of such devices in both conventional and novel architectures.